Copied from upstream nettle git repository.
Removed changes to ChangeLog, to allow this patch to apply to nettle-3.5.

From 485b5e2820a057e873b1ba812fdb39cae4adf98c Mon Sep 17 00:00:00 2001
From: Niels Möller <nisse@lysator.liu.se>
Date: Mon, 17 May 2021 20:55:26 +0200
Subject: [PATCH 1/2] Change _rsa_sec_compute_root_tr to take a fix input size.

Improves consistency with _rsa_sec_compute_root, and fixes zero-input bug.
---
 ChangeLog                    | 15 +++++++++
 rsa-decrypt-tr.c             |  7 ++---
 rsa-internal.h               |  4 +--
 rsa-sec-decrypt.c            |  9 ++++--
 rsa-sign-tr.c                | 61 +++++++++++++++++-------------------
 testsuite/rsa-encrypt-test.c | 14 ++++++++-
 6 files changed, 68 insertions(+), 42 deletions(-)

diff --git a/rsa-decrypt-tr.c b/rsa-decrypt-tr.c
index 0224c0b7..927a8915 100644
--- a/rsa-decrypt-tr.c
+++ b/rsa-decrypt-tr.c
@@ -52,14 +52,13 @@ rsa_decrypt_tr(const struct rsa_public_key *pub,
   mp_size_t key_limb_size;
   int res;
 
-  key_limb_size = NETTLE_OCTET_SIZE_TO_LIMB_SIZE(key->size);
+  key_limb_size = mpz_size(pub->n);
 
   TMP_GMP_ALLOC (m, key_limb_size);
   TMP_GMP_ALLOC (em, key->size);
+  mpz_limbs_copy(m, gibberish, key_limb_size);
 
-  res = _rsa_sec_compute_root_tr (pub, key, random_ctx, random, m,
-				  mpz_limbs_read(gibberish),
-				  mpz_size(gibberish));
+  res = _rsa_sec_compute_root_tr (pub, key, random_ctx, random, m, m);
 
   mpn_get_base256 (em, key->size, m, key_limb_size);
 
diff --git a/rsa-internal.h b/rsa-internal.h
index b828e451..f66a7df0 100644
--- a/rsa-internal.h
+++ b/rsa-internal.h
@@ -78,11 +78,11 @@ _rsa_sec_compute_root(const struct rsa_private_key *key,
                       mp_limb_t *scratch);
 
 /* Safe side-channel silent variant, using RSA blinding, and checking the
- * result after CRT. */
+ * result after CRT. In-place calls, with x == m, is allowed. */
 int
 _rsa_sec_compute_root_tr(const struct rsa_public_key *pub,
 			 const struct rsa_private_key *key,
 			 void *random_ctx, nettle_random_func *random,
-			 mp_limb_t *x, const mp_limb_t *m, size_t mn);
+			 mp_limb_t *x, const mp_limb_t *m);
 
 #endif /* NETTLE_RSA_INTERNAL_H_INCLUDED */
diff --git a/rsa-sec-decrypt.c b/rsa-sec-decrypt.c
index 6866e7c8..fc4757a0 100644
--- a/rsa-sec-decrypt.c
+++ b/rsa-sec-decrypt.c
@@ -58,9 +58,12 @@ rsa_sec_decrypt(const struct rsa_public_key *pub,
   TMP_GMP_ALLOC (m, mpz_size(pub->n));
   TMP_GMP_ALLOC (em, key->size);
 
-  res = _rsa_sec_compute_root_tr (pub, key, random_ctx, random, m,
-				  mpz_limbs_read(gibberish),
-				  mpz_size(gibberish));
+  /* We need a copy because m can be shorter than key_size,
+   * but _rsa_sec_compute_root_tr expect all inputs to be
+   * normalized to a key_size long buffer length */
+  mpz_limbs_copy(m, gibberish, mpz_size(pub->n));
+
+  res = _rsa_sec_compute_root_tr (pub, key, random_ctx, random, m, m);
 
   mpn_get_base256 (em, key->size, m, mpz_size(pub->n));
 
diff --git a/rsa-sign-tr.c b/rsa-sign-tr.c
index f824c4ca..9e137c7a 100644
--- a/rsa-sign-tr.c
+++ b/rsa-sign-tr.c
@@ -131,35 +131,34 @@ int
 _rsa_sec_compute_root_tr(const struct rsa_public_key *pub,
 			 const struct rsa_private_key *key,
 			 void *random_ctx, nettle_random_func *random,
-			 mp_limb_t *x, const mp_limb_t *m, size_t mn)
+			 mp_limb_t *x, const mp_limb_t *m)
 {
+  mp_size_t nn;
   mpz_t mz;
   mpz_t xz;
   int res;
 
-  mpz_init(mz);
   mpz_init(xz);
 
-  mpn_copyi(mpz_limbs_write(mz, mn), m, mn);
-  mpz_limbs_finish(mz, mn);
+  nn = mpz_size (pub->n);
 
-  res = rsa_compute_root_tr(pub, key, random_ctx, random, xz, mz);
+  res = rsa_compute_root_tr(pub, key, random_ctx, random, xz,
+			    mpz_roinit_n(mz, m, nn));
 
   if (res)
-    mpz_limbs_copy(x, xz, mpz_size(pub->n));
+    mpz_limbs_copy(x, xz, nn);
 
-  mpz_clear(mz);
   mpz_clear(xz);
   return res;
 }
 #else
 /* Blinds m, by computing c = m r^e (mod n), for a random r. Also
-   returns the inverse (ri), for use by rsa_unblind. */
+   returns the inverse (ri), for use by rsa_unblind. Must have c != m,
+   no in-place operation.*/
 static void
 rsa_sec_blind (const struct rsa_public_key *pub,
                void *random_ctx, nettle_random_func *random,
-               mp_limb_t *c, mp_limb_t *ri, const mp_limb_t *m,
-               mp_size_t mn)
+               mp_limb_t *c, mp_limb_t *ri, const mp_limb_t *m)
 {
   const mp_limb_t *ep = mpz_limbs_read (pub->e);
   const mp_limb_t *np = mpz_limbs_read (pub->n);
@@ -177,15 +176,15 @@ rsa_sec_blind (const struct rsa_public_key *pub,
 
   /* c = m*(r^e) mod n */
   itch = mpn_sec_powm_itch(nn, ebn, nn);
-  i2 = mpn_sec_mul_itch(nn, mn);
+  i2 = mpn_sec_mul_itch(nn, nn);
   itch = MAX(itch, i2);
-  i2 = mpn_sec_div_r_itch(nn + mn, nn);
+  i2 = mpn_sec_div_r_itch(2*nn, nn);
   itch = MAX(itch, i2);
   i2 = mpn_sec_invert_itch(nn);
   itch = MAX(itch, i2);
 
-  TMP_GMP_ALLOC (tp, nn + mn + itch);
-  scratch = tp + nn + mn;
+  TMP_GMP_ALLOC (tp, 2*nn  + itch);
+  scratch = tp + 2*nn;
 
   /* ri = r^(-1) */
   do
@@ -198,9 +197,8 @@ rsa_sec_blind (const struct rsa_public_key *pub,
   while (!mpn_sec_invert (ri, tp, np, nn, 2 * nn * GMP_NUMB_BITS, scratch));
 
   mpn_sec_powm (c, rp, nn, ep, ebn, np, nn, scratch);
-  /* normally mn == nn, but m can be smaller in some cases */
-  mpn_sec_mul (tp, c, nn, m, mn, scratch);
-  mpn_sec_div_r (tp, nn + mn, np, nn, scratch);
+  mpn_sec_mul (tp, c, nn, m, nn, scratch);
+  mpn_sec_div_r (tp, 2*nn, np, nn, scratch);
   mpn_copyi(c, tp, nn);
 
   TMP_GMP_FREE (r);
@@ -208,7 +206,7 @@ rsa_sec_blind (const struct rsa_public_key *pub,
   TMP_GMP_FREE (tp);
 }
 
-/* m = c ri mod n */
+/* m = c ri mod n. Allows x == c. */
 static void
 rsa_sec_unblind (const struct rsa_public_key *pub,
                  mp_limb_t *x, mp_limb_t *ri, const mp_limb_t *c)
@@ -299,7 +297,7 @@ int
 _rsa_sec_compute_root_tr(const struct rsa_public_key *pub,
 			 const struct rsa_private_key *key,
 			 void *random_ctx, nettle_random_func *random,
-			 mp_limb_t *x, const mp_limb_t *m, size_t mn)
+			 mp_limb_t *x, const mp_limb_t *m)
 {
   TMP_GMP_DECL (c, mp_limb_t);
   TMP_GMP_DECL (ri, mp_limb_t);
@@ -307,7 +305,7 @@ _rsa_sec_compute_root_tr(const struct rsa_public_key *pub,
   size_t key_limb_size;
   int ret;
 
-  key_limb_size = NETTLE_OCTET_SIZE_TO_LIMB_SIZE(key->size);
+  key_limb_size = mpz_size(pub->n);
 
   /* mpz_powm_sec handles only odd moduli. If p, q or n is even, the
      key is invalid and rejected by rsa_private_key_prepare. However,
@@ -321,19 +319,18 @@ _rsa_sec_compute_root_tr(const struct rsa_public_key *pub,
     }
 
   assert(mpz_size(pub->n) == key_limb_size);
-  assert(mn <= key_limb_size);
 
   TMP_GMP_ALLOC (c, key_limb_size);
   TMP_GMP_ALLOC (ri, key_limb_size);
   TMP_GMP_ALLOC (scratch, _rsa_sec_compute_root_itch(key));
 
-  rsa_sec_blind (pub, random_ctx, random, x, ri, m, mn);
+  rsa_sec_blind (pub, random_ctx, random, c, ri, m);
 
-  _rsa_sec_compute_root(key, c, x, scratch);
+  _rsa_sec_compute_root(key, x, c, scratch);
 
-  ret = rsa_sec_check_root(pub, c, x);
+  ret = rsa_sec_check_root(pub, x, c);
 
-  rsa_sec_unblind(pub, x, ri, c);
+  rsa_sec_unblind(pub, x, ri, x);
 
   cnd_mpn_zero(1 - ret, x, key_limb_size);
 
@@ -357,17 +354,17 @@ rsa_compute_root_tr(const struct rsa_public_key *pub,
 		    mpz_t x, const mpz_t m)
 {
   TMP_GMP_DECL (l, mp_limb_t);
+  mp_size_t nn = mpz_size(pub->n);
   int res;
 
-  mp_size_t l_size = NETTLE_OCTET_SIZE_TO_LIMB_SIZE(key->size);
-  TMP_GMP_ALLOC (l, l_size);
+  TMP_GMP_ALLOC (l, nn);
+  mpz_limbs_copy(l, m, nn);
 
-  res = _rsa_sec_compute_root_tr (pub, key, random_ctx, random, l,
-				  mpz_limbs_read(m), mpz_size(m));
+  res = _rsa_sec_compute_root_tr (pub, key, random_ctx, random, l, l);
   if (res) {
-    mp_limb_t *xp = mpz_limbs_write (x, l_size);
-    mpn_copyi (xp, l, l_size);
-    mpz_limbs_finish (x, l_size);
+    mp_limb_t *xp = mpz_limbs_write (x, nn);
+    mpn_copyi (xp, l, nn);
+    mpz_limbs_finish (x, nn);
   }
 
   TMP_GMP_FREE (l);
diff --git a/testsuite/rsa-encrypt-test.c b/testsuite/rsa-encrypt-test.c
index 87525f78..d3bc374b 100644
--- a/testsuite/rsa-encrypt-test.c
+++ b/testsuite/rsa-encrypt-test.c
@@ -19,6 +19,7 @@ test_main(void)
   uint8_t after;
 
   mpz_t gibberish;
+  mpz_t zero;
 
   rsa_private_key_init(&key);
   rsa_public_key_init(&pub);
@@ -101,6 +102,17 @@ test_main(void)
   ASSERT(decrypted[decrypted_length] == after);
   ASSERT(decrypted[0] == 'A');
 
+  /* Test zero input. */
+  mpz_init_set_ui (zero, 0);
+  decrypted_length = msg_length;
+  ASSERT(!rsa_decrypt(&key, &decrypted_length, decrypted, zero));
+  ASSERT(!rsa_decrypt_tr(&pub, &key,
+			 &lfib, (nettle_random_func *) knuth_lfib_random,
+			 &decrypted_length, decrypted, zero));
+  ASSERT(!rsa_sec_decrypt(&pub, &key,
+			  &lfib, (nettle_random_func *) knuth_lfib_random,
+			  decrypted_length, decrypted, zero));
+  ASSERT(decrypted_length == msg_length);
 
   /* Test invalid key. */
   mpz_add_ui (key.q, key.q, 2);
@@ -112,6 +124,6 @@ test_main(void)
   rsa_private_key_clear(&key);
   rsa_public_key_clear(&pub);
   mpz_clear(gibberish);
+  mpz_clear(zero);
   free(decrypted);
 }
-  
-- 
2.31.1

